This invention relates to a signal receiver, especially a receiver for pulse modulated signals. Pulse modulated signals are signals in which parameter of a pulse train is varied in accordance with data.
In many conventional signal receivers a received signal is demodulated to extract a pulse modulated signal. The pulses of the pulse modulated signal represent symbols by means of which the message data are encoded. These symbols have to be decoded to determine the underlying message data. In the section of the receiver that processes the pulse modulated signal there is a postdetection filter for filtering the pulse modulated signal that is input to that section, for example to remove high frequency noise, and a decoder for decoding the signal that is output from the filter. The signal input to the postdetection filter has typically suffered already from degradation. This arises from noise, co-channel interference, intersymbol interference (ISI) and distortion from components earlier in the receiver circuit. Therefore, postdetection filters are normally selected to minimise any further degradation. The postdetection filter is usually chosen to be a Bessel filter or a Butterworth filter because these have a generally linear phase response, which leads to little distortion of the pulses in the signal output from the filter, and frequency responses of increasing loss at increasing frequency, which is felt to remove as much noise as possible. FIG. 1 illustrates at 1 the frequency response of a typical Bessel filter. FIG. 2 illustrates at 2 the phase response of a typical Bessel filter as a plot of relative group delay against frequency. Because the pulse modulated signal is degraded, techniques such as DFE (decision feedback equalisation) and Viterbi decoding are used in the decoder to increase the likelihood of correctly decoding the signal output from the postdetection filter.